Re: [PATCH] hugepage: allow parallelization of the hugepage faultpath

[David Gibson]

On Thu, Jul 18, 2013 at 05:42:35PM +0900, Joonsoo Kim wrote:
> On Wed, Jul 17, 2013 at 12:50:25PM -0700, Davidlohr Bueso wrote:
> > From: David Gibson <david@xxxxxxxxxxxxxxxxxxxxx>
> > 
> > At present, the page fault path for hugepages is serialized by a
> > single mutex. This is used to avoid spurious out-of-memory conditions
> > when the hugepage pool is fully utilized (two processes or threads can
> > race to instantiate the same mapping with the last hugepage from the
> > pool, the race loser returning VM_FAULT_OOM).  This problem is
> > specific to hugepages, because it is normal to want to use every
> > single hugepage in the system - with normal pages we simply assume
> > there will always be a few spare pages which can be used temporarily
> > until the race is resolved.
> > 
> > Unfortunately this serialization also means that clearing of hugepages
> > cannot be parallelized across multiple CPUs, which can lead to very
> > long process startup times when using large numbers of hugepages.
> > 
> > This patch improves the situation by replacing the single mutex with a
> > table of mutexes, selected based on a hash, which allows us to know
> > which page in the file we're instantiating. For shared mappings, the
> > hash key is selected based on the address space and file offset being faulted.
> > Similarly, for private mappings, the mm and virtual address are used.
> > 
> 
> Hello.
> 
> With this table mutex, we cannot protect region tracking structure.
> See below comment.
> 
> /*
>  * Region tracking -- allows tracking of reservations and instantiated pages
>  *                    across the pages in a mapping.
>  *
>  * The region data structures are protected by a combination of the mmap_sem
>  * and the hugetlb_instantion_mutex.  To access or modify a region the caller
>  * must either hold the mmap_sem for write, or the mmap_sem for read and
>  * the hugetlb_instantiation mutex:
>  *
>  *      down_write(&mm->mmap_sem);
>  * or
>  *      down_read(&mm->mmap_sem);
>  *      mutex_lock(&hugetlb_instantiation_mutex);
>  */

Ugh.  Who the hell added that.  I guess you'll need to split of
another mutex for that purpose, afaict there should be no interaction
with the actual, intended purpose of the instantiation mutex.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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